Police were investigating how the weapon fired and Olden said “there are several fail-safes in place” to prevent a discharge. They had not determined how the weapon went off, she said, adding that one safety measure did engage.

“One fail-safe was effective in that the bullet disintegrated when discharged by striking a metal plate where the barrel rests against the weapons mount,” Olden said in a statement.

Two students were taken to an area hospital with injuries not believed to be life-threatening, said Chino Valley Fire District spokeswoman Massiel Ladrón De Guevara. The third student’s parents arrived at the school and declined to have paramedics transport the child to the hospital, she said.

One student who was injured was released from the hospital Wednesday night after metal was removed from the child’s eye, KTLA-TV reported.

A parent told the station that her son was playing four-square when he heard the shot and saw the gun smoking on the motorcycle. Kids were crying and scared, and some didn’t want to return to school, she said her son told her.

“Why was it loaded and why wasn’t the safety on?” she said.

Comments on the Chino Valley Unified School District Facebook page questioned why the AR-15 was loaded and how the student had access to it. Others said their children were afraid to go back to school.

Law enforcement officials were at the school for activities related to a nationwide drug abuse prevention campaign when the weapon was discharged during a demonstration, according to a statement from district.

Red Ribbon Week activities will continue at the school as planned while police and school officials continue to investigate the incident, district spokeswoman Julie Gobin said.